1. Field of the Invention
The present invention relates to a control device for an electric power steering apparatus, and more particularly concerns such a control device for an electric power steering apparatus that achieves a superior steering performance by providing a smooth steering assist force.
2. Description of the Related Art
In an electric power steering apparatus for use in a vehicle, a steering torque and a vehicle speed generated by a steering shaft when a steering wheel is operated are detected, and based upon the detected signals, a steering assist instruction value is calculated so that, based upon the calculated steering assist instruction value, a motor is driven so as to assist the steering force of the steering wheel; thus, in order to calculate the steering assist instruction value and to control the motor based upon the steering assist instruction value, an electronic control circuit including a microcomputer is used.
Here, with respect to the steering torque, there are a component corresponding to a road-surface load generated in response to the operation of the steering wheel and a component corresponding to a frictional force exerted by a steering mechanism. For this reason, a system has been proposed, in which a control value corresponding to the road surface load determined based upon the detected steering torque and a control value corresponding to the frictional force of the steering mechanism are added so as to obtain the steering assist instruction value.
In the above control device, road-face load control values in association with steering torques and frictional force control values in association with the steering torques are preliminarily determined respectively, and stored in a memory; thus, in response to a detected steering torque, the corresponding data is read from the memory so that the steering assist instruction value is calculated (for example, Japanese Patent Publication No. 5-10271 B2).
In such a conventional control means for calculating the steering assist instruction value, the road-face load control value and the frictional force control value have been preliminarily determined in accordance with the steering torque; therefore, once the steering torque has been determined, the steering assist instruction value is allowed to vary only in response to the vehicle speed.
Therefore, the steering assist instruction values corresponding to the steering torques are preliminarily set with respect to a plurality of vehicle speeds so that, based upon a detected steering torque and vehicle speed, it is possible to directly obtain the steering assist value. By setting the steering assist instruction values corresponding to the vehicle speed and the steering torque in a step manner, it becomes possible to reduce the memory capacity; however, since the steering assist instruction value does not change continuously in response to changes in the vehicle speed and the steering torque, the steering assist force is not allowed to change smoothly, resulting in degradation in the sense of steering.
In order to solve the above problems, the steering assist instruction value can be set as finely as possible in accordance with the vehicle speed and the steering torque. However, this method causes a great increase in the required memory capacity, and the subsequent high costs.
In order to solve the above problems, a method has been proposed by the applicant of the present invention (Japanese Patent Application Laid-open No. 8-150954 A), in which: with respect to typical vehicle speeds, steering assist instruction values corresponding to steering torques are stored in a memory, and in the case when a determination is made in such a manner that the detected vehicle speed is located between the typical vehicle speeds stored in the memory, the steering assist instruction values corresponding to the steering torques with respect to the typical vehicle speeds before and after the detected vehicle speed are read from the storage means, and based upon the differences from the detected vehicle speed and vehicle speed correction coefficients, a steering assist instruction value corresponding to the detected vehicle speed and steering torque is calculated.
However, even in the above method in which, with respect to the typical vehicle speeds, the steering assist instruction values corresponding to the steering torques are stored in the memory, when the steering assist instruction values corresponding to the steering torques are set finely, there is still an increase in the required memory capacity and the resulting increase in costs. In addition, in the case when the steering assist instruction value is altered, another problem arises in that time consuming tasks are required since the stored data in the memory needs to be altered. Still another problem is that when the steering assist instruction value corresponding to the steering torque is calculated, that is, when, for example, the calculation is carried out using a finite word length of 8 bits, the lower digits (8 bits) of the results of calculation (having 16 bits) are rounded off, resulting in a quantization error due to digital operations. Such a quantization error causes the driver to have an undesired discontinuous sense of the steering when a gentle steering operation is carried out.
For this reason, another method has been proposed by the applicant of the present invention (Japanese Patent Application Laid-open No. 10-59203 A), in which: a steering assist instruction value operation means carries out operations based upon a plurality of approximation functional expressions so as to make an approximation to a high-order function expression that is coincident with at least three points in the steering assist instruction value corresponding to the steering torque.
However, in the above-mentioned operation method using a plurality of approximation functional expressions so as to make an approximation to a high-order function expression, the resulting function is formed by simply connecting a plurality of quadratic or more functions; therefore, although a smooth steering assist instruction value is obtained except for the connection sections of the functions, the rate of change in the function becomes discontinuous in the connection sections, resulting in degradation in the sense of steering. Moreover, in attempts to stabilize the mechanical system of the electric power steering apparatus, to stabilize vibration in the reduction gear section by using a rubber damper and to adjust the sense of steering, at a step after the steering assist instruction value operation unit of the control apparatus, a control system is used, in which a characteristic gain curve of the entire control system with the steering assist instruction value as an input at least partially has a gradient that exceeds a predetermined value (for example, 10 dB/dec). In this case, however, since the control system has a differential characteristic, and since one-step differentiation of the functions is discontinuous at the connecting sections of the functions of quadratic or more, the output of the control system becomes a discontinuous state or the like, resulting in further degradation in the sense of steering.
The present invention has been devised to solve the above-mentioned problems, and its object is to provide a control device for an electric power steering apparatus which achieves a continuous and pleasant sense of steering by supplying a continuous function as a steering assist instruction value of the electric power steering apparatus.
The present invention relates to a control device for an electric power steering apparatus, which controls a motor based upon a steering assist instruction value calculated in a steering assist instruction operation section based upon a steering torque generated in a steering shaft and a current control value calculated from a current value of the motor for applying a steering assist force to a steering mechanism, and has a gain curve of characteristics of the entire control system having the steering assist instruction value as its input, which at least partially possesses a gradient exceeding a predetermined value, and the object of the present invention is achieved by an arrangement in which: the steering assist instruction value operation unit is composed of an approximation functional expression in which at least one-step derived functions are continuous, and the steering assist instruction value is obtained through calculations based upon said approximation functional expression.
Moreover, the object of the present invention is achieved more effectively by providing the above-mentioned approximation functional expression as a spline function or a quadratic spline function.